In color photographic materials, silver halide emulsions are employed which are sensitized to specific regions of the visible spectrum commonly referred to as: the blue region (400–500 nm), green region (500–600 nm), and red region (600–700 nm), using different chemical sensitizing dyes that are adsorbed to the surface of the silver halide emulsion grains. Color negative photographic films are usually designed to record complementary color images using separate emulsion layers that have distinct sensitivities to various regions of the visible spectrum. For example, color photographic elements are conventionally formed with superimposed blue, green, and red recording layer units coated on a support. The blue, green, and red recording layer units contain radiation-sensitive silver halide emulsions that form a latent image in response to blue, green, and red light, respectively. Additionally, the blue recording layer unit typically contains a yellow dye-forming coupler, the green recording layer unit typically contains a magenta dye-forming coupler, and the red recording layer unit typically contains a cyan dye-forming coupler. However, in addition to their desired light sensitivity, these emulsions also have an inherent sensitivity to UV radiation. In order to achieve accurate color reproduction, it is generally desired that only visible light be incident on the photosensitive emulsion layers during camera exposure.
The blue sensitivity of a multilayer film element is determined by the light absorption profile of the silver halide emulsions in the blue sensitive layer unit attenuated by any ultraviolet light absorbing materials that lie above it in the top layers of the film, such as ultraviolet filter dyes, Lippmann emulsions, and polymeric beads used to reduce friction in the top layers of the film. The light absorption of the emulsions used in the blue sensitive layer unit is in turn determined by the composite absorption of the specific combination of spectral sensitizing dyes adsorbed to the surface of the silver halide grains and the intrinsic blue light absorption of silver bromide and silver iodide.
Conventional non-tabular silver (iodo)bromide emulsions having grains which are primarily cubic, octahedral, cubo-octahedral or polymorphic in shape typically have an inherent sensitivity to visible light in the region of about 400–430 nm, and accordingly may be used in a blue recording layer unit with or without spectral sensitizing dyes. Tabular grain emulsions are also known for use in the blue sensitive layer of color negative film elements. Tabular grains, when present in the blue sensitive layer, result in improved transmission of incident light to underlying green and red sensitive layers. In order to provide practical photographic efficiency, however, such grains typically need to be sensitized in the blue region by a spectral dyeing technique to yield blue sensitive emulsions, as high tabularity, low bulk iodide tabular grains have relatively little inherent sensitivity in the 400–500 nm range. By spectrally sensitizing these emulsions where there exists a higher number of photons per unit energy (that is, between 450–480 nm of the blue region), the sensitivity and hence the efficiency of the blue sensitive record containing these elements is maximized. Optionally, a blue sensitizing dye, or combination of dyes, can also be used with conventional nontabular grains to provide sensitivity to the 430–500 nm region of the visible spectrum. Silver halide grains employed in green and red light recording layers, whether tabular or non-tabular, require sensitizing dyes on the emulsion grains to sensitize them to the required red and green region of the spectrum.
Following imagewise exposure, a negative working photographic element is processed in a color developer that contains a color developing agent that is oxidized while selectively reducing to silver the latent image bearing silver halide grains. The oxidized color developing agent then reacts with the dyeforming coupler in the vicinity of the developed grains to produce an image dye. Yellow (blue-absorbing), magenta (green-absorbing) and cyan (red-absorbing) image dyes are formed in the blue, green, and red recording layer units, respectively. Subsequently the element is bleached (i.e., developed silver is converted back to silver halide) to eliminate neutral density attributable to developed silver and then fixed (i.e., silver halide is removed) to provide stability during subsequent room light handling.
When processing is conducted as noted above, negative dye images are produced. To produce corresponding positive dye images, and hence, to produce a visual approximation of the hues of the subject photographed, white light is typically passed through the color negative image to expose a second color photographic material having blue, green, and red recording layer units as described above, usually coated on a white reflective support. The second element is commonly referred to as a color print element. Processing of the color print element as described above produces a viewable positive image that approximates that of the subject originally photographed.
UV absorbing materials are commonly used in color photographic film products. The preparation and use of UV absorbers has been described in U.S. Pat. Nos. 3,813,255, 4,611,061, and 5,385,815 and European Published Applications 0,057,160 and 0,190,003. They are usually located in layers above and below the light-sensitive emulsion layers. UV absorbers are used below the emulsion layers to prevent static marking from the base side due to exposure from electrical discharge that may be generated during the winding and unwinding of rolled film during manufacturing (both sensitizing and finishing) operations or during mechanical winding and unwinding inside a film camera or during film processing.
The UV region of the electromagnetic spectrum is approximately 200 to 400 nanometers. Since any electrical discharge would take place in air, which is mostly nitrogen, the spectral emission of a “spark” is characteristic of nitrogen, and has strong bands in the 300–400 nm region of the spectrum. UV absorbers are also coated above the emulsion layers for spark protection from the front side as previously described and to prevent UV radiation from striking the emulsion layers during camera lens exposure, as UV radiation reflected from a photographic subject or scene may interfere with the accurate reproduction of color in the finished print.
In choosing materials to be used as UV absorbers above the emulsion layers, it is generally desired that they have a strong absorption throughout the longer UV region (300–400 nm) with little or no absorption in the visible region (400–700 nm). If the absorption of the material does not extend throughout the UV region, then exposure of the emulsions to UV light may adversely impact color reproduction. If the absorption of the material extends too far into the visible region (e.g., >400 nm), on the other hand, then light sensitivity (photographic speed) of the photographic element in the blue region of the spectrum will be reduced. Hence, the ideal UV absorber would absorb sufficient UV radiation to prevent color reproduction error, while minimizing absorption of visible light. It is also desirable to use materials that are inexpensive and have a high extinction coefficient (UV absorbing power) so that low amounts of material may be employed.
There are many organic compounds available that absorb UV radiation at shorter wavelengths (<about 370 nm), but many of these have low extinction coefficients, requiring them to be used at high levels in the coated layers to provide adequate protection. The use of these materials alone with insufficient absorption at longer wavelengths results in two problems. First of all, the spark sensitivity may be too high. Secondly, too much UV radiation would be incident on the emulsion layers, resulting in inaccurate color reproduction.
There are also many organic materials that absorb UV at longer wavelengths, but most of these materials have absorptions that extend significantly into the visible region of the spectrum. These materials generally provide adequate UV protection, but they reduce the light sensitivity of the photographic element in the blue region. This results in lower effective photographic speed, requiring coating higher levels of blue-sensitive silver halide emulsion, which results in undesirably higher material costs.
Dibenzoylmethane ultraviolet absorbing compounds are known in the art for their use as UV light blockers in cosmetic sunscreens as described in, e.g., U.S. Pat. Nos. 4,387,089; 5,783,178; 5,849,273; 5,788,954; 5,993,789; and 6,129,909. Additionally, Wu et. al “A Study of the Photo-stabilizing Behaviors of β-Diketones,” Polymer Degradation Stability, (16) 1986, 169–186 discloses the use of dibenzoylmethane derivatives as UV exposure stabilizers for viscosity retention of polybutadiene solutions. Further, Japanese Kokai JP 04213348 A2 discloses the use of a dibenzoylmethane derivative to improve the weatherability of an acrylic resin film by incorporating the UV absorber inside of cross-linked polymer particles that are dispersed in the resin film to reduce migration out of the film. U.S. Pat. Nos. 6,872,766 and 6,767,937 teach the use of mixtures of dibenzoylmethane and other ultraviolet ray absorbing compounds molecularly dispersed within a polymeric film such as those fabricated into polarizers for use in displays. Dibenzoylmethane derivatives are also contemplated as antihalation agents in photothermographic imaging elements as disclosed in copending, commonly assigned U.S. Ser. No. 10/896,108 filed Jul. 21, 2004. Such compounds have not been previously suggested, however, for use in an ultraviolet filter layer coated above a silver halide emulsion layer in a photographic element.